Semiconductor device and method for producing the same

ABSTRACT

A method of producing a seminconductor device is disclosed in which, after proton implantation is performed, a hydrogen-induced donor is formed by a furnace annealing process to form an n-type field stop layer. A disorder generated in a proton passage region is reduced by a laser annealing process to form an n-type disorder reduction region. As such, the n-type field stop layer and the n-type disorder reduction region are formed by the proton implantation. Therefore, it is possible to provide a stable and inexpensive semiconductor device which has low conduction resistance and can improve electrical characteristics, such as a leakage current, and a method for producing the semiconductor device.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to a semiconductor device including afield stop (FS) layer, such as a diode and an insulated gate bipolartransistor (IGBT), and a method for producing the same.

B. Description of the Related Art

As a power semiconductor device, for example, there is a diode or anIGBT with a breakdown voltage of 400 V, 600 V, 1200 V, 1700 V, 3300 V,or more. The diode or the IGBT is used in a power conversion apparatussuch as a converter or an inverter. The power semiconductor devicerequires good electrical characteristics, such as low loss, highefficiency, and a high breakdown voltage, and low costs.

As a method for producing the power semiconductor device, the followingmethod has been proposed. First, for example, a diffusion region or aMOS structure is formed on the front surface side of a semiconductorsubstrate. Then, the rear surface is ground to reduce the thickness ofthe semiconductor substrate. Proton implantation and a heat treatmentare performed for the ground surface to form donors using compositedefects including the implanted hydrogen atoms and a plurality ofneighboring point defects, thereby forming a high-concentration n-typefield stop layer. The donor formed by the composite defect includinghydrogen is referred to as a hydrogen-induced donor.

Patent Literature 1 discloses a technique for preventing a reduction inelectron/hole mobility at an irradiation position due to protonimplantation. Patent Literature 2 discloses heat treatment conditionsafter proton implantation. Patent Literature 3 discloses an IGBTproduction method which uses laser annealing when a contact layer isformed after proton implantation and annealing. After the protonirradiation, annealing is performed to recover carrier concentration.Patent Literature 4 discloses a method which recovers a defect layerbefore proton annealing to increase the carrier concentration ofprotons. Patent Literature 5 discloses a method which introduces oxygento a silicon substrate in advance, radiates protons to the frontsurface, performs annealing, grinds the rear surface, implantsphosphorus ions into the ground surface, and performs annealing with aYAG laser. In addition, Patent Literature 5 discloses a method whichprevents a reduction in the carrier mobility of a proton-implantedregion due to the introduction of oxygen. Patent Literature 6 disclosesa method which performs annealing with a YAG laser and a CW laser toform a proton field stop layer (proton donor generation layer) afterprotons are implanted into the rear surface.

CITATION LIST

-   Patent Literature 1: United States Patent Application, Publication    No. 2005/0116249-   Patent Literature 2: United States Patent Application, Publication    No. 2006/0286753-   Patent Literature 3: JP 2001-160559 A-   Patent Literature 4: JP 2009-099705 A-   Patent Literature 5: WO 2007-055352 A1-   Patent Literature 6: JP 2009-176892 A

A large number of defects introduced by proton implantation remain inthe projected range Rp of protons (the distance of a position where theconcentration of the implanted ions is the highest from an implantationsurface), a proton passage region from the implantation surface to theprojected range Rp, or the vicinity of the implantation surface. Theremaining defects are referred to as residual defects. The deviation ofan atom (in this case, silicon) from a lattice position is large and thestate of the defect is close to an amorphous state due to the strongdisorder of the crystal lattice. Therefore, the defect is the scatteringcenter of a carrier, such as an electron or a hole, reduces carriermobility, and increases conduction resistance. In addition, the defectis the generation center of the carrier and increases a leakage current.As such, the defect causes the deterioration of the characteristics ofthe element.

As such, the defect which remains in the proton passage region from theproton implantation surface to the projected range Rp of the protons dueto proton implantation, causes a reduction in carrier mobility or anincrease in leakage current, and is strongly disordered from the crystalstate is particularly referred to as disorder. There is a method thatrecovers the crystal defects which are generated during protonimplantation using a heat treatment in an electric furnace to formhydrogen-induced donors. In a case in which the crystal defectsgenerated during proton implantation form the disorder, when only theheat treatment using the electric furnace is performed, thehydrogen-induced donors are formed and the disorder remains in theproton passage region. As a result, the carrier mobility is reduced,which causes deterioration of characteristics, such as an increase inleakage current or conduction loss.

As disclosed in Patent Literature 3, there is a method which anneals theproton implantation surface with a laser while cooling the MOS gateforming surface opposite to the proton implantation surface after protonimplantation is performed. However, Patent Literature 3 does notdisclose the remaining disorder and the influence of the remainingdisorder on the characteristics of the element.

As disclosed in Patent Literature 4, there is a method which performselectron beam heating or laser heating to recover the crystal defectsafter proton implantation, in order to prevent the outward diffusion ofprotons before annealing. However, Patent Literature 4 does not disclosethe remaining disorder and the influence of the remaining disorder onthe characteristics of the element.

As in Patent Literature 5, when a high concentration of oxygen isintroduced into the silicon substrate in advance, a process of diffusingoxygen at a high temperature (1000° C. or more) is needed. Therefore,problems, such as an increase in the number of processes increases andthe occurrence of an oxidation-induced stacking faults (OSF), arise.

In Patent Literature 6, laser beams with two types of wavelengths areradiated to recover the defects in a region from the proton implantationsurface to a depth of 30 μm and a long carrier lifetime is maintained.However, Patent Literature 6 does not disclose the disorder whichremains in the proton passage region. In addition, even when lasers withdifferent wavelengths are combined with each other, a temperaturedistribution is certainly generated in the depth direction. Therefore,it is difficult to achieve both the stable formation of thehydrogen-induced donors at an arbitrary depth and a reduction in thedisorder in the vicinity of the implantation surface and in the passageregion. In addition, individual laser light sources and individual laserirradiation facilities are needed in order to radiate laser beams withtwo different wavelengths, which results in an increase in costs.

In the method disclosed in Patent Literature 6, when the projected rangeRp of the implanted protons from the implantation surface is more than15 μm, the disorder is not sufficiently reduced in the vicinity of theimplantation surface and in the passage region. FIG. 6 is acharacteristic diagram illustrating the comparison among the carrierconcentration distributions of the semiconductor device which isproduced by the method according to the related art for each projectedrange. Specifically, In FIG. 6, when the projected range Rp of protonimplantation is around 15 μm, the carrier concentration distributions ofthe sample which is formed by the method disclosed in Patent Literature6 for each projected range Rp are compared. FIG. 6( a) shows a case inwhich the projected range Rp is 50 μm, FIG. 6( b) shows a case in whichthe projected range Rp is 20 μm, and FIG. 6( c) shows a case in whichthe projected range Rp is 10 μm. In FIG. 6( c) in which the projectedrange Rp is 10 μm, the carrier concentration in the vicinity of theimplantation surface (a depth of 0 μm to 5 μm) and in the passage regionis higher than the concentration, 1×10¹⁴ (/cm³), of the siliconsubstrate and the disorder is sufficiently reduced. As can be seen fromFIG. 6( b) in which the projected range Rp is 20 μm and FIG. 6( a) inwhich the projected range Rp is 50 μm, the carrier concentration in thevicinity of the implantation surface and in the passage region isgreatly reduced and the disorder is not reduced. As such, when thedisorder remains, the leakage current or conduction loss of the elementincreases. Therefore, when the projected range Rp of the protonimplantation is more than 15 μm, a new method for reducing the disorderneeds to be examined.

The present invention is directed to overcoming or at least reducing theeffects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

The invention provides a stable and inexpensive semiconductor devicewhich includes an n-type field stop layer formed at a predetermineddepth, can reduce the disorder generated by proton implantation, and canprevent deterioration of electrical characteristics, such as a reductionin carrier mobility, an increase in loss, and an increase in leakagecurrent due to a generation center, and a method for producing the same,in order to solve the above-mentioned problems.

A semiconductor device according to the invention has the followingcharacteristics. A p-type base layer is provided in a surface layer of afirst main surface of a semiconductor substrate, which is an n-typedrift layer, and has a higher concentration than the drift layer. Ann-type emitter layer is provided in the base layer and has a higherconcentration than the base layer. A gate insulating film is providedwhich contacts the base layer, the emitter layer, and the drift layer. Agate electrode is provided on a surface of the gate insulating filmfacing the base layer, the emitter layer, and the drift layer. Anemitter electrode is provided on the surfaces of the emitter layer andthe base layer and is insulated from the gate electrode by an interlayerinsulating film. A p-type collector layer is provided on a second mainsurface of the semiconductor substrate. At least one n-type intermediatelayer is provided between the drift layer and the collector layer andincludes a pair of an n-type field stop layer with a higherconcentration than the drift layer and an n-type disorder reductionregion with a concentration which is lower than that of the field stoplayer and equal to or higher than that of the drift layer.

In the semiconductor device according to the invention, the depth of aposition where carrier concentration is the maximum in the field stoplayer closest to the base layer from the second main surface may be morethan 15 μm.

In the semiconductor device according to the invention, when q is anelementary charge, N_(d) is the average concentration of the driftlayer, ε_(s) is the permittivity of the semiconductor substrate,V_(rate) is a rated voltage, J_(F) is rated current density, and v_(sat)is a saturated velocity in which a carrier speed is saturated withpredetermined electric field intensity, a distance index L may berepresented by the following Expression (1). When the depth of theposition where the carrier concentration is the maximum in the fieldstop layer closest to the base layer from the second main surface is Xand the thickness of the semiconductor substrate is W0, X=W0−γL may beestablished and γ may be in the range of 0.2 to 1.5.

$\begin{matrix}{L = \sqrt{\frac{ɛ_{S}V_{rate}}{\left( {\frac{J_{F}}{{qv}_{sat}} + N_{d}} \right)}}} & {{Expression}\mspace{14mu} (1)}\end{matrix}$

In the semiconductor device according to the invention, the field stoplayer in the intermediate layer which comes into contact with the driftlayer may come into contact with the drift layer. The disorder reductionregion in the intermediate layer which comes into contact with thecollector layer may come into contact with the collector layer.

In the semiconductor device according to the invention, two or moreintermediate layers may be formed.

In the semiconductor device according to the invention, a minimum valueof carrier mobility in the disorder reduction region may be equal to ormore than 20% of the carrier mobility in a crystal state.

In order to solve the above-mentioned problems, a method for producing asemiconductor device according to the invention has the followingcharacteristics. Proton implantation is performed from one main surfaceof a semiconductor substrate. Then, the entire semiconductor substrateis heated at a high temperature to form a hydrogen-induced donor usingthe proton implantation, thereby forming an n-type field stop layer inthe semiconductor substrate. A portion of the semiconductor substratecorresponding to the projected range of the implanted protons from theone main surface is heated to reduce a disorder generated in a protonpassage region, thereby forming an n-type disorder reduction region.

In the method for producing a semiconductor device according to theinvention, a process of heating the entire semiconductor substrate at ahigh temperature may be a furnace annealing process, and the field stoplayer may be formed by the furnace annealing process. In addition, aprocess of heating the portion of the semiconductor substratecorresponding to the projected range of the implanted protons from theone main surface may be a laser annealing process of radiating laserlight to the one main surface. The disorder reduction region may beformed by the laser annealing process.

In the method for producing a semiconductor device according to theinvention, the formation of the field stop layer and the formation ofthe disorder reduction region may be performed after a rear surface ofthe semiconductor substrate is ground and before a rear electrode isformed.

In the method for producing a semiconductor device according to theinvention, the formation of the field stop layer may be performed afterthe proton implantation is performed and before the disorder reductionregion is formed.

In the method for producing a semiconductor device according to theinvention, the formation of the disorder reduction region may beperformed after the proton implantation is performed and before thefield stop layer is formed.

In the method for producing a semiconductor device according to theinvention, the temperature of the furnace annealing process may be equalto or higher than 350° C. and equal to or lower than 550° C. Theprocessing time of the furnace annealing process may be equal to or morethan 1 hour and equal to or less than 10 hours.

In the method for producing a semiconductor device according to theinvention, a YAG laser or a semiconductor laser may be used in the laserannealing process.

In the method for producing a semiconductor device according to theinvention, after the proton implantation and before the laser annealingprocess, impurity ions may be implanted into a surface layer of the onemain surface which is shallower than the of the implanted protons andthe impurity may be activated by the laser annealing process.

In the method for producing a semiconductor device according to theinvention, when the field stop layer with a Rp of the implanted protonsis formed, the acceleration energy E of the protons may satisfy thefollowing Expression (2).

y=−0.0047x ⁴+0.0528x ³−0.2211x ²+0.9923x+5.0474.  Expression (2)

A semiconductor device according to the invention is an IGBT includingthe field stop layer.

A semiconductor device according to the invention is an FWD includingthe field stop layer.

According to the invention, after the proton implantation, thehydrogen-induced donor is formed by the furnace annealing process toform the n-type field stop layer. In addition, the disorder generated inthe proton passage region is reduced by the laser annealing process toform the disorder reduction region. The formation of the disorderreduction region makes it possible to prevent deterioration ofelectrical characteristics, such as an increase in the conductionresistance or leakage current of a semiconductor device. As a result, itis possible to provide a stable and inexpensive semiconductor device anda method for producing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages and features of the invention will becomeapparent upon reference to the following detailed description and theaccompanying drawings, of which:

FIG. 1 is a process flow diagram illustrating a method for producing asemiconductor device (IGBT 100) according to Embodiment 1 of theinvention.

FIG. 2 is a diagram illustrating carrier concentration profiles 12 and13 in the vicinity of an n-type field stop layer 3 when a laserannealing process is performed and when the laser annealing process isnot performed, respectively.

FIG. 3A is a cross-sectional view illustrating a main portion of theIGBT 100 produced by the process flow of FIG. 1 and FIG. 3B is a diagramillustrating the carrier concentration profile in the vicinity of then-type field stop layer 3.

FIG. 4 is a process flow diagram illustrating a method for producing asemiconductor device according to Embodiment 2 of the invention.

FIG. 5A is a cross-sectional view illustrating a main portion of asemiconductor device (IGBT 100) according to Embodiment 3 of theinvention and FIG. 5B is a diagram illustrating the carrierconcentration profile in the vicinity of an n-type field stop layer 3.

FIGS. 6A, 6B and 6C are characteristic diagrams illustrating the carrierconcentration distributions of a semiconductor device produced by amethod according to the related art for each projected range.

FIG. 7 is a characteristic diagram illustrating a threshold voltage atwhich a voltage waveform starts to oscillate.

FIG. 8 is a diagram illustrating the turn-off oscillation waveform of ageneral IGBT.

FIG. 9 is a characteristic diagram illustrating the relationship betweenthe projected range of protons and the acceleration energy of theprotons in the semiconductor device according to the invention.

FIG. 10 is a table illustrating the position conditions of a field stop(FS) layer which a depletion layer reaches first in the semiconductordevice according to the invention.

FIGS. 11A, 11B and 11C are graphs illustrating the comparison among thecarrier concentration distributions of a sample which is formed only byfurnace annealing for each projected range of proton implantation whenthe projected range is around 15 μm.

FIGS. 12A and 12B are cross-sectional views illustrating an IGBTaccording to the invention.

FIGS. 13A and 13B are cross-sectional views illustrating a diodeaccording to the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, a semiconductor device and a method for producing the sameaccording to exemplary embodiments of the invention will be described indetail with reference to the accompanying drawings. In the specificationand the accompanying drawings, in the layers or regions having “n” or“p” appended thereto, an electron or a hole means a majority carrier.That is, “n” indicates an n type and “p” indicates a p type. In thedescription of the following embodiments and the accompanying drawings,the same components are denoted by the same reference numerals and thedescription thereof will not be repeated.

Embodiment 1

FIG. 1 is a process flow diagram illustrating a method for producing asemiconductor device (IGBT 100) according to Embodiment 1 of theinvention. FIG. 3A is a cross-sectional view illustrating a main portionof the IGBT 100 produced by the process flow shown in FIG. 1 and FIG. 3Bis a diagram illustrating a carrier concentration profile in thevicinity of an n-type field stop layer 3. Next, the method for producingthe semiconductor device according to Embodiment 1 of the invention willbe described with reference to the process flow of FIG. 1 and thecross-sectional view of FIG. 3A.

First, in a front surface forming process shown in (a) of FIG. 1, a MOSgate structure including, for example, a p base layer 22, an n emitterlayer 2, a gate insulating film 23, a gate electrode 24, and aninterlayer insulating film 28 is formed on one main surface (frontsurface 11 a) of an n semiconductor substrate (wafer) 11. However, the nemitter layer 2 is formed in the p base layer 22. The configuration ofthe MOS gate structure will be described below.

Then, in a front electrode process shown in (b) of FIG. 1, an emitterelectrode 25, which is a metal film that commonly comes into conductivecontact with the surfaces of both the p base layer 22 and the n emitterlayer 2 and is also a surface electrode, is formed. Examples of themetal material forming the emitter electrode 25 include Al (aluminum),an Al alloy, such as Al—Si (silicon) or Al—Cu (copper), Cu, Au (gold),and Ni (nickel).

Then, in a surface protective film forming process shown in (c) of FIG.1, a surface protective film which protects a front surface 11 a isformed. The surface protective film is made of a protective tape, aphotoresist, or polyimide. Then, in a rear surface grinding processshown in (d) of FIG. 1, a rear surface 11 b of the n semiconductorsubstrate 11 (n drift layer 1) is ground (back grinding) to apredetermined thickness which is determined by the relationship with abreakdown voltage.

In a proton implantation process shown in (e) of FIG. 1, protonimplantation 16 is performed from a rear surface 11 b. The irradiationenergy of the proton implantation 16 is selected to form the n-typefield stop layer 3 at a predetermined depth. The projected range Rp ofthe implanted protons from the rear surface 11 b (ground surface) isdeeper than 15 μm. In some cases, the projected range Rp is in theprojected range of 20 μm to 400 μm, depending on the rated voltage.Specifically, the projected range Rp of the implanted proton from therear surface 11 b (the ground surface) is, for example, in the projectedrange of 20 μm to 60 μm. The depth of the FS layer, which is formed atthe deepest position from the rear surface 11 b, according to the ratedvoltage will be described below.

In a furnace annealing process shown in (f) of FIG. 1, furnace annealingis performed to the n-type field stop layer 3 using the change ofprotons into donors. The furnace annealing process puts a wafer into aconstant-temperature furnace, such as an electric furnace with aconstant temperature and performs a heat treatment to heat the entirewafer. The processing temperature of the furnace annealing process is inthe range of 350° C. to 550° C. and the processing time is in the rangeof 1 to 10 hours. In the temperature range, crystal defects remain inthe projected range Rp of protons and a proton passage region 14 and theformation of hydrogen-induced donors by the proton implantation 16 isaccelerated. In addition to the furnace annealing, for example, lampannealing may be used to heat the entire wafer. In the temperaturerange, disorder which is introduced by the proton implantation 16 alsoremains in the proton passage region 14 (a region from a protonimplantation surface to the projected range Rp) to form a disorderregion 15. The remaining disorder is the generation center of carrierswhich cause a reduction in the mobility of the carriers or a leakagecurrent.

In a boron implantation process shown in (g) of FIG. 1, boron ions areimplanted into the rear surface 11 b. The boron ion implantation isperformed for a surface layer of the rear surface 11 b such that theprojected range (Rp) of the protons from the rear surface is shallow.

In a laser annealing process shown in (h) of FIG. 1, laser light isradiated to the rear surface 11 b for which the proton implantation 16has been performed to reduce the disorder which remains in the furnaceannealing, thereby forming an n-type disorder reduction region 18, andto activate the boron implanted into the shallow portion from the rearsurface 11 b, thereby forming a p collector layer 4. The laser annealingprocess heats the projected range Rp of the protons radiated to the rearsurface 11 b using, for example, a YAG laser. The pulse width (halfwidth) of the laser light is in the range of, for example, 300 ns to 800ns and the laser light is radiated to the same portion several times.The temperature of silicon in the irradiation portion is about 3000° C.,but a heating time and a cooling time are very short (an order of 10 nsto 1 μs).

The penetration depth of the laser light is within the penetration depth(projected range Rp) of the protons. Specifically, the wavelength of thelaser light is selected such that the penetration depth is from theposition of the base (the end 3 a of the n-type field stop layer 3 closeto the rear surface 11 b) of a mountain (convex portion) with the peakof donor generation concentration (carrier concentration) by theprotons, which is close to the rear surface 11 b, to a protonimplantation surface (rear surface 11 b) (a portion of the protonpassage region 14). It is preferable that the wavelength of the laserlight be in the range of 10 μm to 1000 μm. In this case, it is possibleto effectively perform the formation of the n-type disorder reductionregion 18 and the activation of the boron at the same time. Thetemperature of the portion irradiated with the laser light is equal toor higher than 1000° C. In this case, it is possible to effectively formthe n-type disorder reduction region 18. It is preferable that thetemperature be equal to or higher than 2000° C. and equal to or lowerthan the boiling point (3266° C.) of Si. In particular, the temperatureis not higher than the melting point of silicon. In this case, it ispossible to prevent the ablation (surface roughness) of the irradiationsurface after laser irradiation. In addition, a lamp annealing processmay be performed, instead of the laser annealing process. Theabove-mentioned process makes it possible to form the n-type disorderreduction region 18 between the proton implantation surface and theprojected range (Rp).

In a rear electrode forming process shown in (i) of FIG. 1, a collectorelectrode (not shown) which comes into conductive contact with thesurface of the p collector layer 4 is formed by, for example, vacuumsputtering. In this way, the IGBT 100 according to the invention iscompleted. After the sputtering is performed, a metal annealing processis performed, if necessary. As such, the point of the production methodaccording to this embodiment is to perform both the furnace annealingprocess and the laser annealing process. The effect of this method willbe described below.

FIG. 11 shows graphs illustrating the comparison among the carrierconcentration distributions for each projected range Rp of the protonimplantation which is around 15 μm when a sample is formed only byfurnace annealing. FIG. 11A shows a case in which the projected range Rpis 50 μm, FIG. 11B shows a case in which the projected range Rp is 20μm, and FIG. 11C shows a case in which the projected range Rp is 10 μm.The furnace annealing temperature is 400° C. As shown in FIG. 11, whenonly the furnace annealing is performed, the carrier concentration isgreatly reduced at a depth of about 5 μm from the implantation surfacein any projected range Rp. Thus, as can be seen from FIG. 11, when onlythe furnace annealing is performed, a disorder remains at a depth ofabout 5 μm from the implantation surface, and it is difficult to recovercarrier mobility in a region in the vicinity of the implantation surfacewhen only the furnace annealing is performed.

As described above, when the laser annealing process is performed afterthe furnace annealing process for changing the protons into donors isperformed, it is possible to effectively form the hydrogen-induceddonors and reduce the disorder which remains in the furnace annealing.It is possible to form the n-type disorder reduction region 18. On theother hand, the furnace annealing process alone is insufficient toreduce disorder and the laser annealing process alone is insufficient toform the hydrogen-induced donor. FIG. 2 shows the verification resultsof the carrier concentration profile in the vicinity of the n-type fieldstop layer 3 when the laser annealing process is performed and when thelaser annealing process is not performed.

FIG. 2 shows carrier concentration profiles 12 and 13 in the vicinity ofthe n-type field stop layer 3 when the laser annealing process isperformed and when the laser annealing process is not performed,respectively. After proton implantation is performed for the siliconsubstrate, the furnace annealing process (here, 400° C.) is performed.Then, the disorder which remains in the furnace annealing is reduced bythe laser annealing process to form the n-type disorder reduction region18. The carrier concentration profile 12 shown in FIG. 2 is obtainedafter the n-type disorder reduction region 18 is formed. For comparison,the carrier concentration profile 13 when the laser annealing process isnot performed and the disorder remains is represented by a dotted line.The projected range Rp of protons is 25 μm and the correspondingacceleration energy of the protons is 1.35 MeV. The energy density ofthe laser annealing process is 2.8 J/cm².

In the carrier concentration profile 13 when the laser annealing processis not performed, a disorder region 15 is spread in a region (a portionof the proton passage region 14) between the base (that is, the end 3 aof the n-type field stop layer 3 close to the rear surface 11 b) of thepeak (mountain), which is the projected range 17 (Rp) of the proton, tothe proton implantation surface (rear surface 11 b). The end 3 a of thebase of the peak (mountain) close to the rear surface 11 b is at a depthof about 5 μm from the implantation surface. When the disorder region 15is formed, the mobility of electrons and holes in the region is reduced.Therefore, apparently, the carrier concentration of the disorder region15 is reduced (the resistance thereof increases). In addition, thedisorder is the generation center of carriers and the leakage currentincreases when a voltage is applied.

In the carrier concentration profile 12 when the laser annealing processis performed, the carrier concentration of a portion of the protonpassage region 14 from the implantation surface (rear surface 11 b) tothe end 3 a of the n-type field stop layer 3 close to the rear surface11 b is substantially equal to the concentration of the siliconsubstrate which is deeper than the projected range 17. That is, thedisorder which remains in the proton passage region 14 when only thefurnace annealing process is performed is reduced by both the laserannealing process and the furnace annealing process. The region in whichthe disorder is reduced is referred to as the n-type disorder reductionregion 18. In addition, a pair of one n-type field stop layer 3 and onen-type disorder reduction region 18 which is adjacent to theimplantation surface is referred to as an n-type intermediate layer 27.

Next, the difference between the n-type disorder reduction region 18 inthe carrier concentration profile 12 when the laser annealing process isperformed and the disorder region 15 in the carrier concentrationprofile 13 when the laser annealing process is not performed will bedescribed. In FIG. 2, as described above, it is apparent that thecarrier concentration is reduced in the disorder region 15. That is, inthe carrier concentration profile 13 when the laser annealing process isnot performed which is represented by a dotted line in FIG. 2, it isapparent that the carrier concentration is reduced in the disorderregion 15 before the n-type field stop layer 3 is formed (at a positionshallower than the n-type field stop layer 3 from the rear surface). Theapparent reduction in the carrier concentration is caused by aconversion method which calculates resistivity from the spreadingresistance measured by the known spreading resistance method (SR method)and converts the resistivity into carrier concentration. That is, in thedisorder region 15 in which a crystal lattice is disordered, electronsor holes are scattered by strong disorder, which results in a reductionin the mobility of the carriers.

In general, in the measurement device, a theoretical value in a crystalstate is set to the value of carrier mobility. Therefore, thecalculation of carrier concentration is affected by a reduction in themobility of carriers by the disorder region 15 as follows. For example,the electron mobility of silicon at room temperature (about 300K) is1360 (cm²/V·s) and the hole mobility thereof is 495 (cm²/V·s). Themobility of carriers does not contribute to converting spreadingresistance into resistivity. A conversion expression for convertingresistivity into carrier concentration is represented by ρ=1/(μ·q·N)(where N is carrier concentration (cm⁻³) and p is resistivity (Ω·cm)).In addition, μ is the mobility of electrons or holes (cm²/V·s) and q isan elementary charge (1.6×10⁻¹⁹ C). In the conversion expression, whenthe resistivity ρ is a constant value ρ_(o) and an ideal mobility valuein the crystal state which is greater than the reduced mobility issubstituted into the position where the reduced mobility will beoriginally substituted, carrier concentration N is calculated to besmall since the resistivity ρ_(o) is a constant value. This appears asan apparent reduction in carrier concentration in the carrierconcentration profile 13 when the laser annealing process is notperformed as shown in FIG. 2. Therefore, as in the invention, when thedisorder of the disorder region 15 which is formed in the proton passageregion 14 on the side of the rear surface 11 b is reduced by the laserannealing process to form the n-type disorder reduction region 18,electrical characteristics, such as a leakage current, are improved.

As described above, the furnace annealing process is needed in order toincrease the donor generation rate of the hydrogen-induced donor. Inaddition to the furnace annealing process, the laser annealing processis needed in order to reduce the disorder which remains in the protonpassage region 14 and to form the n-type disorder reduction region 18.When the temperature of the furnace annealing process is too high, areduction in the disorder generated by proton implantation isaccelerated, but the number of crystal defects required to form thehydrogen-induced donors using protons is insufficient. As a result, thedonor generation rate is reduced. Therefore, the furnace annealingprocess may be performed in the above-mentioned temperature range andthe processing time range.

When energy density in the laser annealing process is too high, ablationoccurs and the rear surface is roughened. It is preferable that the pnjunction plane of the p collector layer be flat. When roughness occurs,the pn junction plane corresponds to the shape of the roughness. On theother hand, when the energy density is two low, the disorder in thevicinity of the implantation surface (a depth of 5 μm from theimplantation surface) is not sufficiently removed, but remains. As aresult, carrier mobility is reduced. The results of the inventors'research proved that the sum of the energy density of the laserirradiation surface in laser annealing was preferably equal to orgreater than 1 J/cm² and equal to or less than 4J/cm².

FIG. 2 verifies to what extent the carrier mobility in the n-typedisorder reduction region 18 can be recovered. In FIG. 2, the carrierconcentration of the n semiconductor substrate 11 is about 6×10¹³(/CM³). In contrast, when the laser annealing process is performed, thecarrier concentration of a portion (a portion of the proton passageregion) from the implantation surface to a depth of 5 μm is about 5×10(/cm³) and is recovered up to 83% of the substrate concentration. Thatis, this shows that the carrier mobility is recovered up to about 83% ofthe theoretical value of a crystal. On the other hand, when the laserannealing process is not performed (only the furnace annealing processis performed), the carrier mobility is about 1.2×10¹³ (/cm³) in theimplantation surface with the lowest carrier concentration and isreduced to about 20%. The minimum value of the carrier mobility in then-type disorder reduction region 18 depends on the reduction state ofconduction loss (the saturated voltage V_(CE)(sat) of the IGBT). Theminimum value of the carrier mobility is equal to or greater than 20% ofthe theoretical value of the crystal state and preferably equal to orgreater than 50% of the theoretical value. In this case, the influenceof the minimum value of the carrier mobility is negligible, as comparedto the conduction loss when the carrier mobility is not reduced. Inaddition, the upper limit of the minimum value of the carrier mobilityin the n-type disorder reduction region 18 is preferably 100% of thetheoretical value of the crystal state.

It is preferable that the temperature of the furnace annealing processbe equal to or higher than 350° C. and equal to or lower than 550° C.When the temperature is lower than 350° C., disorder remains in theentire proton implantation region from the implantation surface to theprojected range Rp and the minimum value of the carrier mobility is lessthan 10%. On the other hand, when the temperature is equal to or higherthan 550° C., the hydrogen-induced donor disappears. The temperature ismore preferably equal to or higher than 380° C. and equal to or lowerthan 450° C., and most preferably equal to or higher than 400° C. andequal to or lower than 420° C. In this case, it is possible to achieveboth the formation of the hydrogen-induced donor and the suppression ofa reduction in carrier mobility.

In some cases, the carrier concentration of the n-type disorderreduction region 18 is lower than the concentration of the n-type fieldstop layer 3 and is higher than the concentration of the n semiconductorsubstrate 11. This is because the hydrogen ion remaining in the protonpassage region 14 and the neighboring point defect form a compositedefect. In this case, when the concentration of the proton passageregion 14 is equal to or higher than the concentration of the nsemiconductor substrate 11, disorder is reduced in the n-type disorderreduction region 18.

The IGBT 100 produced by the process flow shown in FIG. 1 will bedescribed in detail with reference to FIG. 3. As described above, the pbase layer 22 is formed in the front surface 11 a of the n semiconductorsubstrate 11 and the n emitter layer 2 is formed in a surface layer ofthe p base layer 22. The gate electrode 24 is formed on a portion of thep base layer 22 which is interposed between the n emitter layer 2 andthe n semiconductor substrate 11, with the gate insulating film 23interposed therebetween. The gate electrode 24, the gate insulating film23, the n semiconductor substrate 11, and the n emitter layer 2 form theMOS gate structure. The interlayer insulating film 28 is formed on thegate electrode 24 and the emitter electrode 25 which comes intoconductive contact with the n emitter layer 2 and the p base layer 22 isformed on the interlayer insulating film 28.

The n-type field stop layer 3 with a higher carrier concentration thanthe n semiconductor substrate 11 is formed in the rear surface 11 b ofthe n semiconductor substrate 11 by proton implantation and furnaceannealing. The p collector layer 4 and the n-type disorder reductionregion 18 are formed at a position that is shallower than the n-typefield stop layer 3 by boron ion implantation and a laser annealingprocess for reducing disorder. A collector electrode (not shown) isformed on the p collector layer 4. In this way, the IGBT 100 iscompleted. However, the n semiconductor substrate 11 from the n-typefield stop layer 3 to the p base layer 22 is the n drift layer 1 inwhich the main current flows to maintain the main breakdown voltage. Thecross-sectional view of the formed IGBT 100 is shown in FIG. 12. FIG. 12is a cross-sectional view illustrating the IGBT according to theinvention. A leakage stop layer 32 with concentration that is higherthan that of the n-type field stop layer 3 and is lower than that of thep collector layer 4 may be formed between the p collector layer 4 andthe n-type disorder reduction region 18. The leakage stop layer 32 isformed by, for example, phosphorus ion implantation.

As described above, the n-type field stop layer 3 of the IGBT 100 isformed by performing the proton implantation 16 from the rear surface 11b and performing the furnace annealing process such that an appropriatenumber of crystal defects, thereby changing protons into donors. Theformation of the n-type disorder reduction region 18 by a reduction inthe disorder formed by the proton passage region 14 is effectivelyperformed by the laser annealing process of radiating laser light to therear surface 11 b. The IGBT 100 shown in FIG. 3 and the IGBT 100 shownin FIG. 12 have different MOS gate structures (FIG. 3 shows a planargate structure and FIG. 12 shows a trench gate structure). However, whenthe n-type intermediate layer 27 formed by a pair of the n-type fieldstop layer 3 and the n-type disorder reduction region 18 is formed inthe rear surface 11 b, the two structures have the same effect. Thestructure which includes two or more n-type intermediate layers 27 asshown in FIG. 12 will be described below. In FIG. 12, similarly to FIG.3, reference numeral 22 denotes a p base layer, reference numeral 23denotes a gate insulating film, reference numeral 24 denotes a gateelectrode, reference numeral 25 denotes an emitter electrode, andreference numeral 28 denotes an interlayer insulating film. In addition,reference numeral 33 denotes a collector electrode.

Embodiment 2

A semiconductor device according to Embodiment 2 will be described. FIG.4 is a process flow illustrating a method for producing thesemiconductor device according to Embodiment 2 of the invention.

Embodiment 2 differs from Embodiment 1 in that proton implantation isfollowed by boron implantation (a process (e) of FIG. 4) and then alaser annealing process (a process (f) of FIG. 4) and a furnaceannealing process (a process (g) of FIG. 4) are sequentially performed.In this case, in the laser annealing process which is performed first,boron is activated to form the p collector layer 4 at the same time asthe n-type disorder reduction region 18 is formed in the proton passageregion 14 formed by the proton implantation 16. In the furnace annealingprocess, which is a post-process, protons are changed into donors toform the n-type field stop layer 3. In this case, similarly toEmbodiment 1, the n-type disorder reduction region 18 is formed by acombination of the laser annealing process and the furnace annealingprocess and the donor generation rate increases to form the n-type fieldstop layer 3. The formation of the n-type disorder reduction regionmakes it possible to improve electrical characteristics, such as theleakage current of the semiconductor device.

In the process flow shown in FIG. 1, the front surface 11 a opposite tothe proton implantation surface (rear surface 11 b) shown in FIG. 3 iscovered with a protective film, the protective film is removed after theproton implantation 16, and furnace annealing is performed. Then, asecond protective film as a protective film for second boron ionimplantation is formed and laser annealing is performed. That is, theprotective film needs to be formed two times. In contrast, in theprocess flow shown in FIG. 4, after the laser annealing process, theprocess of removing the protective film may be performed only once.Therefore, it is possible to reduce manufacturing costs, as compared tothe process flow shown in FIG. 1.

However, the semiconductor device (IGBT 100) produced by the processflow shown in FIG. 4 has the same structure as that shown in FIG. 3.

Embodiment 3

A semiconductor device according to Embodiment 3 will be described. Thesemiconductor device according to Embodiment 3 is an IGBT including oneor more n-type intermediate layers 27 each including a pair of an n-typefield stop layer 3 and an n-type disorder reduction region 18. Forexample, in this embodiment, three n-type intermediate layers 27 areformed. FIG. 5A is a cross-sectional view illustrating a main portion ofthe semiconductor device (IGBT 100) according to Embodiment 3 of theinvention and FIG. 5B is a diagram illustrating a carrier concentrationprofile in the vicinity of the n-type field stop layer 3. As such, whena plurality of n-type intermediate layers 27 are formed, it is possibleto relax the spreading of a depletion layer when the semiconductordevice is turned off. Therefore, it is possible to suppress anoscillation phenomenon during switching (when the semiconductor deviceis turned off).

Embodiment 4

Next, the preferred position of a first proton peak in a plurality ofproton irradiation operations in a method for producing a semiconductordevice according to Embodiment 4 of the invention will be described.

FIG. 8 shows the turn-off oscillation waveform of a general IGBT. When acollector current is equal to or less than one-tenth of the ratedcurrent, in some cases, the number of carriers accumulated is small andoscillation occurs before the turning-off of the IGBT ends. Thecollector current is fixed to a certain value and the IGBT is turned offby a different power supply voltage V_(CC). In this case, when the powersupply voltage V_(CC) is greater than a predetermined value, it isgreater than the peak value of a general overshoot voltage in thevoltage waveform between the collector and the emitter and an additionalovershoot occurs. The additional overshoot (voltage) serves as a triggerand the subsequent waveform oscillates. When the power supply voltageV_(CC) is greater than the predetermined value, the additional overshootvoltage further increases and the subsequent oscillation amplitude alsoincreases. As such, a threshold voltage at which the voltage waveformstarts to oscillate is referred to as an oscillation start thresholdvalue V_(RRO). When the oscillation start threshold value V_(RRO)increases, the IGBT does not oscillate when it is turned off, which ispreferable.

The oscillation start threshold value V_(RRO) depends on the position ofa first proton peak that a depletion layer (strictly, a space chargeregion since there is a hole), which is spread from the pn junctionbetween a p base layer and an n drift layer of the IGBT to the n driftlayer, reaches first, among a plurality of proton peaks. The reason isas follows. When the depletion layer is spread from the pn junctionbetween the p base layer and the n drift layer to the n drift layer atthe time the IGBT is turned off, the end of the depletion layer reachesthe first field stop (FS) layer (closest to the p base layer) and thespreading of the depletion layer is suppressed. The sweep of theaccumulated carriers is weakened. As a result, the depletion of thecarriers is suppressed and oscillation is suppressed.

When the IGBT is turned off, the depletion layer is spread from the pnjunction between the p base layer and the n drift layer (hereinafter,simply referred to as the pn junction) toward the collector electrode inthe depth direction. Therefore, the peak position of the FS layer whichthe end of the depletion layer reaches first is the FS layer which isclosest to the pn junction. Here, the thickness of the n semiconductorsubstrate (the thickness of a portion interposed between the emitterelectrode and the collector electrode) is W0 and the depth of the peakposition of the FS layer which the end of the depletion layer reachesfirst from the interface between the collector electrode and the rearsurface of the n semiconductor substrate 11 (hereinafter, referred to asa distance from the rear surface) is X. A distance index L isintroduced. The distance index L is represented by the followingExpression (1).

$\begin{matrix}{L = \sqrt{\frac{ɛ_{S}V_{rate}}{\left( {\frac{J_{F}}{{qv}_{sat}} + N_{d}} \right)}}} & {{Expression}\mspace{14mu} (1)}\end{matrix}$

The distance index L represented by the above-mentioned Expression (1)is an index indicating the distance of the end (depletion layer end) ofthe depletion layer (exactly, a space charge region), which is spreadfrom the pn junction to the n drift layer 1, from the pn junction when avoltage V_(CE) between the collector and the emitter is a power supplyvoltage V_(CC) at the time the IGBT is turned off. In a fraction in thesquare root, a denominator indicates the space charge density of thespace charge region (depletion layer) when the IGBT is turned off. Theknown Poisson equation is represented by divE=ρ/ε, in which E iselectric field intensity, ρ is space charge density, andρ=q(p−n+N_(d)−N_(a)) is established. Here, q is an elementary charge, ρis hole concentration, n is electron concentration, N_(d) is donorconcentration, N_(a) is acceptor concentration, and ε_(S) is thepermittivity of a semiconductor. In particular, the donor concentrationN_(d) is average concentration obtained by integrating the n drift layerin the depth direction and dividing the integrated value by the distanceof the integrated section. The net doping concentration shown in FIG.13( b) is the net doping concentration of N_(d)−N_(a) and the axisindicates the absolute value of N_(d)−N_(a).

The space charge density ρ is described by the hole concentration ρpassing through the space charge region (depletion layer) when the IGBTis turned off and the average donor concentration N_(d) of the n driftlayer. The electron concentration is lower than these concentrations soas to be negligible and there is no acceptor. Therefore, ρ≈q(p+N_(d)) isestablished. In this case, the hole concentration p is determined by abreaking current of the IGBT. In particular, since a situation in whichthe element is being energized is assumed, the rated current density ofthe element is expressed as p=J_(F)/(qv_(sat)). In addition, J_(F) isthe rated current density of the element and v_(sat) is a saturatedvelocity in which the speed of the carrier is saturated withpredetermined electric field intensity.

The Poisson equation is integrated with respect to the distance x twotimes and the voltage V satisfies E=−gradV (the relationship between theknown electric field E and the voltage V). Therefore, under appropriateboundary conditions, V=(1/2)(ρ/ε)x² is established. The length x of thespace charge region obtained when the voltage V is half of the ratedvoltage V_(rate) is the distance index L. This is because an operationvoltage (power supply voltage), which is the voltage V, is about half ofthe rated voltage in the actual device such as an inverter. Since thedoping concentration of the FS layer is higher than that of the n driftlayer, the FS layer prevents the expansion of the space charge regionwhich is spread at the time the IBGT is turned off. In a case in whichthe collector current of the IGBT starts to be reduced from the breakingcurrent due to the turn-off of a MOS gate, when the peak position of theFS layer which the depletion layer reaches first is in the length rangeof the space charge region, it is possible to suppress the expansion ofthe space charge region, with the accumulated carriers remaining in then drift layer. Therefore, the sweep of the remaining carriers issuppressed.

In the actual turning-off operation, for example, when an IGBT module isdriven by a known PWM inverter, the power supply voltage or the breakingcurrent is not fixed, but is variable. Therefore, in this case, thepreferred peak position of the FS layer which the depletion layerreaches first needs to have a given width. According to the results ofthe inventors' research, the distance X of the peak position of the FSlayer which the depletion layer reaches first from the rear surface isas shown in FIG. 10. FIG. 10 is a table illustrating the positionconditions of the FS (field stop) layer which the depletion layerreaches first in the semiconductor device according to the invention.FIG. 10 shows the distance X of the peak position of the FS layer whichthe end of the depletion layer reaches first from the rear surface at arated voltage of 600 V to 6500 V. Here, X=W0−γL is established and γ isa coefficient. FIG. 10 shows the distance X when γ is changed from 0.7to 1.6.

As shown in FIG. 10, at each rated voltage, the element (IGBT) is safelydesigned so as to have a breakdown voltage which is about 10% higherthan the rated voltage. As shown in FIG. 10, the total thickness of then semiconductor substrate (the thickness during a finishing processafter the n semiconductor substrate is thinned by, for example,grinding) and the average resistivity of the n drift layer are set suchthat an on-voltage or turn-off loss is sufficiently reduced. The term‘average’ means the average concentration and resistivity of the entiren drift layer including the FS layer. The rated current density has thetypical value shown in FIG. 10 due to the rated voltage. The ratedcurrent density is set such that energy density which is determined bythe product of the rated voltage and the rated current density has asubstantially constant value and has nearly the value shown in FIG. 10.When the distance index L is calculated using these values according theabove-mentioned Expression (1), the value shown in FIG. 10 is obtained.The distance X of the peak position of the FS layer which the end of thedepletion layer reaches first from the rear surface is obtained bysubtracting the product of the distance index L and γ, which is in therange of 0.7 to 1.6, from the thickness W0 of the n semiconductorsubstrate 11.

The coefficient γ for determining the distance X of the peak position ofthe FS layer which the end of the depletion layer reaches first from therear surface, at which turn-off oscillation is sufficiently suppressed,is as follows with respect to the distance index L and the thickness W0of the n semiconductor substrate 11. FIG. 7 is a characteristic diagramillustrating a threshold voltage at which the voltage waveform starts tooscillate. Specifically, FIG. 7 is a graph illustrating the dependenceof V_(RRO) on γ at some typical rated voltages V_(rate) (600 V, 1200 V,and 3300 V). In FIG. 7, the vertical axis indicates a value obtained bystandardizing V_(RRO) with the rated voltage V_(rate). As can be seenfrom FIG. 7, V_(RRO) can be rapidly increased at three rated voltageswhen γ is equal to or less than 1.6.

As described above, in the actual apparatus, such as an inverter, sincethe operation voltage (power supply voltage V_(CC)), which is thevoltage V, is about half of the rated voltage V_(rate). Therefore, whenV_(CC) is half of V_(rate), it is necessary to prevent the occurrence ofat least the turn-off oscillation of the IGBT. That is, the value ofV_(RRO)/V_(rate) needs to be equal to or greater than 0.5. As can beseen from FIG. 7, the value of V_(RRO)/V_(rate) is equal to or greaterthan 0.5 at γ of 0.2 to 1.5. Therefore, it is preferable that γ be inthe range of at least 0.2 to 1.5.

At all of a rate voltage of 600 V to 1200 V (for example, 800 V or 1000V), a rate voltage of 1200 V to 3300 V (for example, 1400 V, 1700 V, or2500 V), and a rate voltage of 3300 V or more (for example, 4500 V or6500 V), the value of V_(RRO)/V_(rate) does not greatly from threecurves and has the same dependence (the value of V_(RRO) with respect toγ). As can be seen from FIG. 7, when γ is in the range of 0.7 to 1.4, itis possible to sufficiently increase V_(RRO) at any rated voltage.

When γ is less than 0.7, the avalanche breakdown voltage of the elementis likely to be lower than the rated voltage since V_(RRO) is equal toor higher than about 80% of the rated voltage, but the FS layer is closeto the p base layer. Therefore, it is preferable that γ be equal to orgreater than 0.7. When γ is greater than 1.4, V_(RRO) is rapidly reducedfrom about 70% and turn-off oscillation is likely to occur. Therefore,it is preferable that γ be equal to or less than 1.4. In addition, γ ismore preferably in the range of 0.8 to 1.3 and most preferably in therange of 0.9 to 1.2. In this case, it is possible to maximize V_(RRO)while sufficiently increasing the avalanche breakdown voltage of theelement to be higher than the rated voltage.

The important point in the effect of the invention shown in FIG. 7 isthat the range of γ capable of sufficiently increasing V_(RRO) issubstantially the same (0.7 to 1.4) at any rated voltage. The reason isthat it is most effective to set the range of the distance X of the peakposition of the FS layer which the depletion layer reaches first fromthe rear surface to be substantially centered on W0−L (that is, γ=1.0).It is most effective to include γ=1.0 since power density (the productof the rated voltage and the rated current density) is substantiallyconstant (for example, 1.8×10⁵ VA/cm² to 2.6×10⁵ VA/cm²). That is, whenthe voltage of the element corresponds to the rated voltage duringswitching, such as turn-off, the distance (depth) of the end of thespace charge region is about the distance index L represented by theabove-mentioned Expression (1). When the peak position of the FS layerwhich is deepest from the rear surface is the position of L (that is, γis about 1.0), it is possible to suppress oscillation during switching.Since the power density is substantially constant, the distance index Lis proportional to the rated voltage V_(rate). Therefore, in the rangehaving γ=1.0 as its substantial center, it is possible to sufficientlyincrease V_(RRO) at any rated voltage V_(rate). As a result, it ispossible to maximize the effect of suppressing oscillation duringswitching.

As described above, when the distance X of the peak position of the FSlayer which the end of the depletion layer reaches first is in theabove-mentioned range, a sufficient number of accumulated carriers canremain in the IGBT at the time the IGBT is turned off, and it ispossible to suppress the oscillation phenomenon at the time the IGBT isturned off. Therefore, for the distance X of the peak position of the FSlayer which the end of the depletion layer reaches first, thecoefficient γ of the distance index L may be in the above-mentionedrange at any rated voltage. In this case, it is possible to effectivelysuppress the oscillation phenomenon when the IGBT is turned off.

As can be seen from FIG. 10, when the depth of the first FS layer, whichis deepest from the rear surface, from the rear surface is about γ=1 ata rated voltage of 600 V or more as described above, the distance indexL is more than a depth of 20 μm at any rated voltage. That is, theprojected range Rp1 of the protons for forming the first proton peak isdeeper than 15 μm from the rear surface of the substrate and is equal toor more than 20 μm in order to maximize the effect of suppressingoscillation.

Embodiment 5

The acceleration energy of protons in a method for producing asemiconductor device according to Embodiment 5 of the invention will bedescribed. The acceleration energy of the protons may be determined fromthe characteristic graph shown in FIG. 9 in order to form an FS layerwhich a depletion layer reaches first and which has a peak position thatis at a distance X from the rear surface, using proton irradiation, suchthat the above-mentioned range of γ is satisfied in practice. FIG. 9 isa characteristic diagram illustrating the relationship between theprojected range of the protons and the acceleration energy of theprotons in the semiconductor device according to the invention.

The results of the inventors' research proved that, for the projectedrange Rp (the peak position of the FS layer) of the protons and theacceleration energy E of the protons, when the logarithm log(Rp) of theprojected range Rp of the protons was x and the logarithm log(E) of theacceleration energy E of the protons was y, x and y satisfied thefollowing relationship represented by Expression (2).

γ=−0.0047x ⁴+0.0528x ³−0.2211x ²+0.9923x+5.0474  Expression (2)

FIG. 9 is a characteristic graph indicating the above-mentionedExpression (2). FIG. 9 shows the acceleration energy of the protons forobtaining the desired projected range of the protons. In FIG. 9, thehorizontal axis is the logarithm log(Rp) of the projected range Rp ofthe protons and indicates the projected range Rp (μm) corresponding tothe parentheses below the axis value of log(Rp). In addition, thevertical axis is the logarithm log(E) of the acceleration energy E ofthe protons and indicates the acceleration energy E of the protonscorresponding to the parentheses on the left side of the axis value oflog(E). The above-mentioned Expression (2) is obtained by fitting thevalue of the logarithm log(Rp) of the projected range Rp of the protonsand the value of the logarithm log(E) of the acceleration energy, whichare obtained by, for example, experiments, with a four-order polynomialof x(=log(Rp)).

The following relationship may be considered between the actualacceleration energy E′ and an average projected range Rp′ (proton peakposition) which is actually obtained by, for example, the spreadingresistance (SR) method when the above-mentioned fitting equation is usedto calculate the acceleration energy E of proton irradiation from thedesired average projected range Rp of the protons and the protons areimplanted into a silicon substrate with the calculated value of theacceleration energy.

When the actual acceleration energy E′ is in the range of about ±10% ofthe calculated value E of the acceleration energy, the actual averageprojected range Rp′ is within the range of about ±10% of the desiredaverage projected range Rp and is in a measurement error range.Therefore, the influence of a variation in the actual average projectedrange Rp′ from the desired average projected range Rp on the electricalcharacteristics of an IGBT is so small to be negligible. When the actualacceleration energy E′ is in the range of ±10% of the calculated valueE, it is possible to determine that the actual average projected rangeRp′ is substantially equal to the set average projected range Rp.Alternatively, when the actual average projected range Rp′ is in therange of about ±10% of the average projected range Rp obtained byapplying the actual acceleration energy E′ to the above-mentionedExpression (2), no problem occurs.

In the actual accelerator, since the acceleration energy E and theaverage projected range Rp are both in the above-mentioned ranges(±10%), it is considered that the actual acceleration energy E′ and theactual average projected range Rp′ follow the above-mentioned fittingequation represented by the desired average projected range Rp and thecalculated value E and no problem occurs. In addition, a variation orerror range may be equal to or less than ±10% of the average projectedrange Rp. When the variation or error range is within the range of ±5%,it can be considered that the actual acceleration energy E′ and theactual average projected range Rp′ follow the above-mentioned Expression(2).

The above-mentioned Expression (2) is used to calculate the accelerationenergy E of the protons required to obtain the desired projected rangeRp of the protons. The acceleration energy E of each proton for formingthe FS layer is also calculated by the above-mentioned Expression (2)and is well matched with the value of a sample measured by the knownspreading resistance measurement method (SR method) after protonirradiation is performed with the above-mentioned acceleration energyE′. Therefore, when the above-mentioned Expression (2) is used, it ispossible to predict the required acceleration energy E of protons withhigh accuracy, on the basis of the projected range Rp of the protons.

Embodiment 6

A semiconductor device according to Embodiment 6 will be described. Theprocess flow shown in FIG. 1 is an example of the process flow forproducing the IGBT 100 and can be applied to a diode. In this case, theprocess (a) of FIG. 1 is a process for forming an anode layer. In theprocess (e) of FIG. 1, boron is replaced with phosphorus or arsenic anda cathode layer is formed by the process (f) of FIG. 1. Thecross-sectional view of the diode which is formed in this way is shownin FIG. 13. FIG. 13 is a cross-sectional view illustrating the diodeaccording to the invention. A p anode layer 52 is formed in the frontsurface of a semiconductor substrate which will be an n drift layer 1and an n cathode layer 53 is formed in the rear surface. Referencenumeral 51 denotes an anode electrode and reference numeral 54 denotes acathode electrode. For example, the position or concentration of a fieldstop layer 3, an n-type disorder reduction region 18, and a n-typeintermediate layer 27 which is a combination of the field stop layer 3and the n-type disorder reduction region 18 is appropriately adjusted,similarly to the IGBT. In Embodiment 6, in the diode, the preferredposition of the first proton peak position among a plurality of protonirradiation operations is the same as that in the IGBT when the turn-offoscillation is replaced with reverse recovery oscillation and theoscillation start threshold value V_(RRO) is the threshold voltage ofthe power supply voltage V_(CC) at which the voltage waveform of thediode starts to oscillate.

In each of the above-described embodiments, various modifications andchanges can be made without departing from the scope and spirit of theinvention. In the above-described embodiments, the hydrogen ionsimplanted into the semiconductor substrate described as protons.However, for example, double hydrogen ions and triple hydrogen ions maybe implanted. The range of the double hydrogen ions and the triplehydrogen ions is shorter than that of protons since the mass thereof isincreased due to a neutron. Therefore, it is preferable to use protonsin order to form the n-type intermediate layer 27 at a deep positionfrom the surface of the semiconductor substrate.

As described above, the semiconductor device and the method forproducing the semiconductor device according to the invention are usefulas power semiconductor devices used for power conversion apparatusessuch as converters or inverters.

Thus, a semiconductor device and a method for producing the same havebeen described according to the present invention. Many modificationsand variations may be made to the techniques and structures describedand illustrated herein without departing from the spirit and scope ofthe invention. Accordingly, it should be understood that the devices andmethods described herein are illustrative only and are not limiting uponthe scope of the invention.

REFERENCE SIGNS LIST

-   -   1 n DRIFT LAYER    -   2 n EMITTER LAYER    -   3 n-TYPE FIELD STOP LAYER    -   3 a END    -   4 p COLLECTOR LAYER    -   11 n SEMICONDUCTOR SUBSTRATE    -   11 a FRONT SURFACE    -   11 b REAR SURFACE    -   12 CARRIER CONCENTRATION PROFILE WHEN LASER ANNEALING PROCESS IS        PERFORMED    -   13 CARRIER CONCENTRATION PROFILE WHEN LASER ANNEALING PROCESS IS        NOT PERFORMED    -   14 PROTON PASSAGE REGION    -   15 DISORDER REGION    -   16 PROTON IMPLANTATION    -   17 PROJECTED RANGE    -   18 n-TYPE DISORDER REDUCTION REGION    -   22 p BASE LAYER    -   23 GATE INSULATING FILM    -   24 GATE ELECTRODE    -   25 EMITTER ELECTRODE    -   27 n-TYPE INTERMEDIATE LAYER    -   28 INTERLAYER INSULATING FILM    -   33 COLLECTOR ELECTRODE    -   100 IGBT

What is claimed is:
 1. A semiconductor device comprising: a p-type baselayer that is provided in a surface layer of a first main surface of asemiconductor substrate, which is an n-type drift layer, and has ahigher concentration than the drift layer; an n-type emitter layer thatis provided in the base layer and has a higher concentration than thebase layer; a gate insulating film that is provided so as to come intocontact with the base layer, the emitter layer, and the drift layer; agate electrode that is provided on a surface of the gate insulating filmso as to face the base layer, the emitter layer, and the drift layer; anemitter electrode that is provided on the surfaces of the emitter layerand the base layer and is insulated from the gate electrode by aninterlayer insulating film; a p-type collector layer that is provided ona second main surface of the semiconductor substrate; and at least onen-type intermediate layer that is provided between the drift layer andthe collector layer and includes a pair of an n-type field stop layerwith a higher concentration than the drift layer and an n-type disorderreduction region with a lower concentration than the field stop layer.2. The semiconductor device according to claim 1, wherein the field stoplayer includes a hydrogen-induced donor.
 3. The semiconductor deviceaccording to claim 1, wherein carrier mobility in the disorder reductionregion is lower than carrier mobility in the drift layer.
 4. Thesemiconductor device according to claim 1, wherein a minimum value ofthe carrier mobility in the disorder reduction region is equal to ormore than 20% of the carrier mobility in a crystal state.
 5. Thesemiconductor device according to claim 1, wherein the dopingconcentration of the disorder reduction region is equal to or higherthan the concentration of the drift layer.
 6. The semiconductor deviceaccording to claim 1, wherein the disorder reduction region includes aresidual disorder which is introduced by hydrogen ion implantation. 7.The semiconductor device according to claim 6, wherein the residualdisorder is generated by reducing the disorder introduced by thehydrogen ion implantation using a heat treatment.
 8. The semiconductordevice according to claim 1, wherein the depth of a position wherecarrier concentration is the maximum in the field stop layer which isadjacent to the base layer, with the drift layer interposedtherebetween, from the second main surface is more than 15 μm.
 9. Thesemiconductor device according to claim 1, wherein, when q is anelementary charge, N_(d) is the average concentration of the driftlayer, ε_(s) is the permittivity of the semiconductor substrate,V_(rate) is a rated voltage, J_(F) is rated current density, and v_(sat)is a saturated velocity in which a carrier speed is saturated withpredetermined electric field intensity, a distance index L isrepresented by the following Expression (1): $\begin{matrix}{L = \sqrt{\frac{ɛ_{S}V_{rate}}{\left( {\frac{J_{F}}{{qv}_{sat}} + N_{d}} \right)}}} & {{Expression}\mspace{14mu} (1)}\end{matrix}$ and when the depth of the position where the carrierconcentration is the maximum in the field stop layer which is adjacentto the base layer, with the drift layer interposed therebetween, fromthe second main surface is X and the thickness of the semiconductorsubstrate is W0, X=W0−γL is established and γ is in the range of 0.2 to1.5.
 10. The semiconductor device according to claim 1, wherein thefield stop layer in the intermediate layer which comes into contact withthe drift layer comes into contact with the drift layer, and thedisorder reduction region in the intermediate layer which comes intocontact with the collector layer comes into contact with the collectorlayer.
 11. The semiconductor device according to claim 1, wherein two ormore intermediate layers are formed.
 12. A method for producing asemiconductor device comprising: a step of performing protonimplantation from one main surface of a semiconductor substrate; a stepof heating the entire semiconductor substrate at a high temperature toform a hydrogen-induced donor using the proton implantation, therebyforming an n-type field stop layer in the semiconductor substrate; and astep of heating a portion of the semiconductor substrate correspondingto the range of the implanted protons from the one main surface toreduce a disorder generated in a proton passage region, thereby formingan n-type disorder reduction region.
 13. The method for producing asemiconductor device according to claim 12, wherein, in the step offorming the field stop layer, a process of heating the entiresemiconductor substrate at a high temperature is a furnace annealingprocess, and in the step of forming the disorder reduction region, aprocess of heating the portion of the semiconductor substratecorresponding to the range of the implanted protons from the one mainsurface is a laser annealing process of radiating laser light to the onemain surface.
 14. The method for producing a semiconductor deviceaccording to claim 12, wherein the range of the implanted protons isequal to or more than 15 μm.
 15. The method for producing asemiconductor device according to claim 13, wherein the disorder isreduced from the one main surface to a depth of 5 μm or more by thelaser annealing process.
 16. The method for producing a semiconductordevice according to claim 12, wherein the disorder causes carriermobility in the proton passage region to be lower than that in a regionother than the proton passage region in the semiconductor substrate. 17.The method for producing a semiconductor device according to claim 13,wherein carrier mobility in the proton passage region is increased bythe furnace annealing process and the laser annealing process.
 18. Themethod for producing a semiconductor device according to claim 12,wherein the step of forming the field stop layer and the step of formingthe disorder reduction region are performed after a rear surface of thesemiconductor substrate is ground and before a rear electrode is formed.19. The method for producing a semiconductor device according to claim12, wherein the step of forming the field stop layer is performed afterthe step of performing the proton implantation and before the step offorming the disorder reduction region.
 20. The method for producing asemiconductor device according to claim 12, wherein the step of formingthe disorder reduction region is performed after the step of performingthe proton implantation and before the step of forming the field stoplayer.
 21. The method for producing a semiconductor device according toclaim 13, wherein the temperature of the furnace annealing process isequal to or higher than 350° C. and equal to or lower than 550° C., andthe processing time of the furnace annealing process is equal to or morethan 1 hour and equal to or less than 10 hours.
 22. The method forproducing a semiconductor device according to claim 13, wherein a YAGlaser or a semiconductor laser is used in the laser annealing process.23. The method for producing a semiconductor device according to claim13, further comprising a step of implanting impurity ions into a surfacelayer of the one main surface which is shallower than the range of theimplanted protons and activating the impurity using the laser annealingprocess after the proton implantation and before the laser annealingprocess.
 24. The method for producing a semiconductor device accordingto claim 12, wherein, when the logarithm log(Rp) of an average range Rpof the implanted protons is x and the logarithm log(E) of theacceleration energy E of the proton implantation is y, the accelerationenergy E of the protons when the field stop layer with the range Rp ofthe implanted protons is formed satisfies the following Expression (2):y=−0.0047x ⁴+0.0528x ³−0.2211x ²+0.9923x+5.0474.  Expression (2)
 25. Asemiconductor device that is produced by the method for producing asemiconductor device according to claim 12, wherein the semiconductordevice is an IGBT including the field stop layer.
 26. A semiconductordevice that is produced by the method for producing a semiconductordevice according to claim 12, wherein the semiconductor device is an FWDincluding the field stop layer.